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Soil
What we call "soil" is actually topsoil. Below the topsoil is subsoil which is fairly humus-free but rich in mineral nutrients. Subsoil "heaviness" determines the drainage of the soil. Below the subsoil is rock. Rock types drain differently and so also affect drainage. !! Contents of soil * Life ** Bacteria ** Mycelium (fungi) ** Yeasts ** Roots and root hairs ** Actinomycetes ** Worms ** Insect larvae * Humus - high molecular weight organic acids * Minerals !! Soil heaviness Soil is classified as "light" or "heavy" based on the size of particles. Sand is the lightest soil (largest particles). | Soil type | heaviness| particle size | pH | | Clay | heavy | very small | usually alkaline | | Silt (loam) | medium | medium | | | Sand | light | very large | usually acidic | The "sausage test" or "rope test" can be used to determine the heaviness of soil. A more detailed test is the "jar test": # Get a jar with a tight-fitting lid, and fill 2/3 with water. # Take some soil, at least a cup, break it up and add to the water. # Tighten the lid and shake vigorously for 1 minute. # Put down and wait 1 minute. # Mark the layer that has settled to the bottom of the jar. This is the sand component of the soil (largest particles). # Wait 1 hour. # Mark the next layer -- this is the silt (loam). # Wait 24 hours. Mark the final layer. This is the clay. !! Soil compaction Soil is compacted when the soil particles are squeezed together, reducing the amount of empty space between them. Well-aerated soil: * Surface water can easily penetrate into the soil where it is absorbed by soil particles. These particles then release the water slowly. * Excess water can drain through the soil easily. * Plant roots can grow through the soil easily. * Plant roots receive adequate oxygen. Compacted soil: * Aeration is poor * Surface water tends to run off the soil surface without penetrating. This leads to topsoil erosion * Drainage of water through the soil is impaired. If the subsoil is compacted, this will lead to waterlogging and reduced aeration * It is hard for plant roots to penetrate the soil, so plant growth is stunted, and plant roots tend to be shallow. This lack of groundcover and root infrastructure exacerbates any soil erosion. * In dry weather, a modest amount of surface compaction can increase plant yields, perhaps because roots are forced deeper into the soil. However yields are reduced with more significant compaction, and with any level of compaction in wet conditions. Causes of soil compaction: * Vehicles (tractors etc) * Excessive tillage can produce a pan of compacted soil below the depth of tillage * Lack of crop rotation -- if only shallow-rooted crops are being planted * Rainfall (minor) * When the soil is wet, the effects of surface impacts are magnified, and compaction extends deeper into the soil. Remedies for compacted soil: * Plant crops with deep roots * Minimise movement over the soil surface (walking, vehicles, livestock etc) !! Soil pH The normal pH of rainwater is not neutral, but rather around 6--6.5, owing to the presence of carbonic acid in the water. Unsurprisingly, most plants also prefer water of the same pH. "Optimum" pH is 6.5--7 {seymour-ssg} Clay is usually alkaline. Sand is usually acidic. The type of rock underlying the soil can influence pH e.g. limestone gives rise to alkaline soil. Acidity can be increased with compost and manure. Alkalinity can be increased with mineral lime. !! Soil organic content This can be quantified by combustion. Thoroughly dry a small sample of soil, then place 10 grams on a pre-weighed dish. Heat it with a propane torch until there are no visible fumes. Allow to cool, then weigh the soil and dish again. !! Resources * http://www.rain.org/global-garden/soil-types-and-testing.htm Category:Resource Category:Concept